1. Field of the Invention
This invention relates generally to the field of overvoltage protection systems for television apparatus, and in particular, to an X-radiation protection circuit which enables television apparatus to operate at ultor voltage levels which are closer to the isoexposure curves of cathode ray tubes.
2. Description of Art
It is normally desirable to operate a picture tube at the highest ultor voltage possible, due to concerns with beam spot size and circuit configuration and limitations in general. One of the limitations on how high the ultor voltage can be is the ultor voltage level as which X-radiation is generated. The Center for Devices and Radiological Health, part of the Food and Drug Administration, sets limits on the maximum permissible X-radiation and a cathode ray tube vendor will specify what the ultor voltage versus beam current relationship is for each particular tube is which will result in generation of X-radiation. This relationship is expressed as an isoexposure curve. An isoexposure curve is shown in a graph in FIG. 3. The isoexposure curve represents X-radiation at a level of 0.5 milliRankins per hour (mR/Hr). The Y axis is the ultor, or high voltage level designated HV. The X axis is beam current (I.sub.BEAM), denoted in milliamps (ma). For the curve shown, a maximum beam current level of approximately 2 ma exceeds the safe level defined by the isoexposure curve at approximately 38 KV. In accordance with the safety rules, a television chassis shall not be allowed to drive a picture tube to any HV vs. I.sub.BEAM point which exceeds the isoexposure curve. It is also required that these excessive conditions cannot exist even with only one fault in the chassis. This fault can occur in any circuit, including for example the high voltage circuit, the X-ray protection (XRP) circuit and the power supply; or can result from any component value change.
FIG. 3 also shows two operating curves. The upper curve is designated with tracking high voltage HV and the lower curve is designated without tracking high voltage HV. The lower operating curve represents operation without benefit of the invention taught herein. The upper operating curve represents operation in accordance with the invention taught herein. Normal operation of the chassis would be at HV vs. I.sub.BEAM levels below the isoexposure curve. A fourth curve in the graph, between the operating curve(s) and the isoexposure curve is the XRP circuit trip curve, that is, the voltage level at which the XRP circuit disables the television in a manner which prevents generation of X-radiation, for example by shutting down the high voltage power supply.
There are tolerances associated with each of these curves which tend to widen the distance between the operating curve(s) and the isoexposure curve. These tolerances are an inherent aspect of essentially all electrical circuits and components, such as the XRP circuit, the high voltage regulator and the operating characteristics for a particular picture tube. There must be enough separation between the isoexposure curve and the operating curve to avoid X-radiation under worst case circumstances of normal operation. This separation is often referred to as the pad.
Ultor voltage is ordinarily generated in a flyback transformer of a resonant retrace horizontal deflection circuit. Such a circuit might generate 29 KV. Some picture tubes require still higher ultor voltages for proper operation. A second resonant retrace circuit having a second flyback transformer can generate a boost voltage, for example 3 KV. The appropriate windings of the flyback transformers of the two resonant retrace circuits are coupled in series to generate an ultor voltage which is the sum of the two voltages, 32 KV in this example. However, with prior art overvoltage protection systems, in which the high voltage from the second circuit was not tracked, it was necessary to add the maximum boost voltage of the high voltage regulator to the amount that the operating curve must be decreased below the isoexposure and XRP trip curves. Such a limitation made it more difficult to operate safely at ultor voltage levels closer to the isoexposure and XRP trip curves, notwithstanding the boost voltage.
The XRP circuit is a circuit detects the value of the kine ultor (HV) voltage and will shut down the HV power supply if the HV reaches a certain voltage. This voltage is determined by the kine X-radiation characteristics. In one known method of operation, a peak detected voltage from the HV transformer is used to indicate the value of HV. This peak detected voltage is not generated by the same winding as the HV, but is on the same transformer and therefore proportional to the HV. In another known method, a HV regulator is used which regulates by inserting a pulse voltage in series with the HV transformer winding which generates HV. Therefore, the high voltage is the peak detected sum of the HV winding voltage and the HV regulator winding voltage. The XRP voltage is not proportional to the HV in this configuration, but is proportional only to the portion of the HV that is generated by the HV transformer. If the isoexposure curves are high enough, such as in certain projection televisions, a large pad can be added to compensate for whatever the HV regulator may boost. Most direct view chassis do not have the luxury of sufficient voltage to allow for this pad between the nominal operating HV and the HV where X-radiation is emitted.